This invention is related to automotive powertrain systems including computer control of various automatic transmission functions. More specifically, the invention is concerned with control of speed flare during downshifts in an automatic transmission.
In a motor vehicle multi-ratio automatic transmission, it is often desirable to effect shifting from one ratio to another ratio without the use of free-wheeling devices such as one way clutches. This requires a coordinated timing control of both off-going and on-coming fluid operated friction elements in order to achieve a desired amount of overlap as the transmitted torque is shifted from the off-going friction element to the on-coming friction element. Typically, the off-going fluid pressure is released through an orifice while the fluid pressure is supplied to the on-coming friction element through a pressure control device such as an accumulator or servo. On-coming friction element engagement rate is controlled by controlling the fluid flow into the accumulator or servo. Examples of such control include varying a restriction at the inlet of the servo such as by pulse width modulation of an electro-hydraulic valve, controlling the average flow rate through a two flow state valve by controlling the proportion of time each flow state is active, or through coordinated control of line pressure and selection of one of two flow states of a solenoid operated valve.
It may be desirable to further adjust the flow rate in the case where the on-coming friction element does not reach a torque capacity sufficient to prevent the transmission input from exceeding the synchronous speed of the on-coming friction element.